The invention relates to a hip cup for use as an acetabular component in a hip prosthesis, comprising a shell part having an at least partially convex outer surface, which shell part can be inserted in a cavity having an inner surface that is substantially defined by the outer surface of a segment of a sphere.
The two techniques that are used most for attaching a hip cup concern the cemented and the uncemented technique respectively. In the cemented technique, the hip cup is fixed by means of bone cement. Within the uncemented technique, screw hip cups and press-fit hip cups are used. Screw cups are screwed into a cavity, which may or may not have been provided artificially in a bone, press-fit hip cups are for instance attached by knocking the hip cup into place in such cavity.
With these methods an attempt is made to approach the original anatomic situation as much as possible through the proper positioning of the acetabular component in the acetabulum. The position of the press-fit hip cup will change slightly in the first few months after positioning, due to the forces applied thereto. This change of position is commonly referred to as "settling".
For applying hip prostheses in patients, it is known to provide in an acetabulum a cavity having a truly spherical inner surface by means of a spherical cutter, wherein, subsequently, a press-fit hip cup is fittingly provided. Hence, this involves the outer surface of the hip cup abutting completely against the inner surface of the cavity formed, as a consequence of which, directly after the positioning of the hip cup, the forces acting on the hip cup are completely distributed on the inner surface of the cavity. The forces applied to the hip cup by the patient, particularly during movements of his leg, will cause the hip cup to slightly change position, especially in the first few months after positioning. Because the bone tissue around the hip cup will not have healed entirely yet in those months, and the hip cup will hence not have integrated entirely in the acetabulum yet, the hip cup will still be capable of moving somewhat, so that space will be created around the longitudinal edge of the hip cup. As the hip cup rests against the apex, it cannot be pressed any further into the cavity and will therefore become increasingly looser due to a cranial displacement, with all its consequences, in particular with regard to the patient's mobility. Moreover, this will cause the occurrence of high local peak loads.